1. Field of the Invention
This invention relates to a process for the monitoring of thin films on a substrate, particularly for the monitoring of a transparent coating applied to a substrate, such as coatings on glass.
Control of the thickness, and also the refractive index, of coatings applied to glass sheets, especially freshly formed ribbons of glass, is important. As is well known in the art, one or more coatings are applied to glass sheets for a number of purposes. These coatings are often thin, for example less than 100 nm, and the measurement of films of this thickness presents difficulties, especially where it is desired to make rapid and continuous measurements over large areas of glass and to detect variations in thickness both along and across the glass ribbon or sheets.
The thickness of these coatings is a very important element in the control of quality of glazing formed from coated glass sheets. The physical and optical properties of the glazing depend strongly on the thickness of the coating. The geometric thickness and the refractive index of the coating play a predominant role in the interference properties of the coated sheet.
If one establishes that the measured value of the thickness of a sample is outside the allowed tolerances, the sheets of glass which have been coated in the meantime will be wasted. This is a particular problem if in the meantime the coated sheets have already been subjected to a further coating or have been made up into glazing panels.
It is therefore desirable to monitor the thickness of the coating as quickly as possible after the deposit of the coating in the industrial production line.
2. Description of the Related Art
British patent Specification GB 2069130 (RCA) describes a method for monitoring the optical thickness of a coating by directing polychromatic light at the sample and successively at a control sample with a coating of known thickness and varying the thickness of the comparison coating until the reflected spectra correspond. This method is suitable for coating optical thicknesses of 150 nm to 3000 nm. The method also requires accurate calibration of the comparison coating.
The process described in GB 2069130 does not permit the control of geometric thicknesses thinner than about 75 nm (given a coating refractive index approaching 2). In addition, the described process is not concerned with the continuous control of coating thicknesses where the measured values fall outside threshold values, nor is it possible with the described apparatus to monitor the uniformity of thickness of the coating. Furthermore, the positioning of the apparatus described in GB 2 069 130 immediately following coating, is difficult.